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Translating Why const Doesn't Make C Code Faster.

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LazyWolf Lin 2019-08-27 13:32:05 +08:00
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@ -39,14 +39,14 @@ void constByArg(const int *x)
} }
``` ```
调用 `printf()` 时，CPU 会通过指针从 RAM 中取得 `*x` 的值。很显然，`constByArg()` 会稍微快一点，因为编译器知道 `*x` 是常量，因此不需要在调用 `constFunc()` 之后再次获取它的值。 It’s just printing the same thing. Right? Let’s see the assembly code generated by GCC with optimisations cranked up: 调用 `printf()` 时，CPU 会通过指针从 RAM 中取得 `*x` 的值。很显然，`constByArg()` 会稍微快一点，因为编译器知道 `*x` 是常量，因此不需要在调用 `constFunc()` 之后再次获取它的值。它仅是打印相同的东西。对吧？让我们来看下 GCC 在如下编译选项下生成的汇编代码：
``` ```
$ gcc -S -Wall -O3 test.c $ gcc -S -Wall -O3 test.c
$ view test.s $ view test.s
``` ```
Here’s the full assembly output for `byArg()`: 以下是函数 `byArg()` 的完整汇编代码：
``` ```
byArg: byArg:
@ -73,7 +73,7 @@ byArg:
.cfi_endproc .cfi_endproc
``` ```
The only difference between the generated assembly code for `byArg()` and `constByArg()` is that `constByArg()` has a `call constFunc@PLT`, just like the source code asked. The `const` itself has literally made zero difference. 函数 `byArg()` 和函数 `constByArg()` 生成的汇编代码中唯一的不同之处是 `constByArg()` 有一句汇编代码 `call constFunc@PLT`，这正是源码中的调用。关键字 `const` 本身并没有造成任何汇编代码上的不同。
Okay, that’s GCC. Maybe we just need a sufficiently smart compiler. Is Clang any better? Okay, that’s GCC. Maybe we just need a sufficiently smart compiler. Is Clang any better?